Self-contained electrical network systems are used frequently on board of a automotive vehicles, boats, or the like, and typically are supplied with power from electrical generators which have a battery connected to the output to supply power when the generator is not operating and, in some cases, also excitation voltage. In typical installations, the generators are three-phase alternators, driven from an internal combustion (IC) engine with a stationary armature. A rotating field is provided, supplied with current over slip rings. The field current is supplied in pulses under control of a voltage regulator. Customary voltage regulators frequently use a semiconductor element, typically a power transistor, to control the current flow through the field winding of the alternator in dependence on output voltage. The output voltage of the alternator is compared with a reference value, for example by use of a Zener diode, which causes the transistor to become conductive if the voltage drops below the reference value and to block the transistor if the voltage exceeds the reference value.
If the controlling element of the voltage regulator, for example the power transistor, or a similar switching element which may be a solid-state switching element, becomes defective and remains continuously conductive, for example, in case of a power transistor if the transistor is short-circuited or alloys, field current is continuously supplied to the field of the alternator causing the generator voltage to rise to excessive levels. The mobile power supply systems have batteries which, if they are charged with voltages in excess of their design or nominal value for an excessive period of time become hot and, due to the constant over-charge, will lose battery fluid and eventually will be damaged or even destroyed by the continued over-charge. The customary indicating systems which are used in automotive application provide for an indication, for example by illumination of a control lamp, that the alternator is not supplying sufficient current to the battery to keep the battery charged and to supply loads thereto; these systems do not, however, indicate to the operator that the voltage regulator became short-circuited and that the battery is being over-charged. Damage to the voltage regulator in the sense of a continuously conductive voltage regulator, is usually not noticed until the electrolyte of the battery is so depleted that the battery already is damaged or destroyed. So long as some electrolyte remains, the various loads connected to the electrical system will operate since the generator continues to supply power. Charge indicator lamps in automotive electrical systems are usually so connected that the positive battery terminal is connected to the positive field terminal and if the field voltage and battery voltage are essentially the same--a condition which pertains when the alternator is charging--the lamp will be extinguished. The operator, therefore, does not have a device which permits monitoring of proper operation of the voltage regulator both with respect to under-charging as well as with respect to overcharging of the battery.